Biomedical Engineering Reference
In-Depth Information
When a nerve impulse (action potential) reaches the axon terminal, it triggers
exocytosis of synaptic vesicles. In this process, the synaptic vesicles fuse with plasma
membrane and release Ach, which diffuses into te synaptic cleft between the motor
neuron and the motor end plate.
On the muscle side of the synaptic cleft, the motor end plate contains acetyl-
choline receptors. These are integral proteins that recognize and bind specifically to
ACh. At the typical NMJ, there are 30-40 million ACh receptors. The binding of
the ACh to its receptor opens a channel that passes small cations—most importantly
Na
. The resulting change in resting membrane potential triggers a muscle action
potential that travels along the muscle cell membrane (sarcolemma) and initiates the
events leading to muscle contraction. In most skeletal muscle fibers, there is only
one NMJ for each muscle fiber located near the fiber's midpoint. The muscle action
potential spreads from the center of the fiber toward both ends. This arrangement
permits nearly simultaneous contraction of all parts of the fiber.
A typical skeletal muscle consists of hundreds or thousands of very long, cylin-
drical cells called muscle fibers (fig. A.4). The muscle fibers lie parallel to one
another and range from 10 to 100
+
µ
m in diameter. While a typical length is 100
µ
m,
some are up to 30 cm long.
The sarcolemma is a muscle fiber's plasma membrane, and it surrounds the
muscle fiber's cytoplasm or sarcoplasm. Because skeletal muscle fibers arise from
the fusion of many smaller cells during embryonic development, each fiber has many
nuclei to direct synthesis of new proteins. The nuclei are at the periphery of the cell
next to the sarcolemma and conveniently out of the way of the contractile elements.
The mitochondria (energy packs) lie in rows throughout the muscle fiber strategically
close to muscle proteins that use ATP to carry on the contraction process. Within
the muscle fibers are myofibrils, which are extended lengthwise in the sarcoplasm.
Their prominent light and dark band colors, called cross-striations, make the whole
muscle fiber appear striped or striated.
Myofibrils form the contractile element of the skeletal muscle (fig. A.5). They
are 1-2
m in diameter and contain three types of smaller filaments called myofil-
aments. These are thin, thick, or elastic filaments. The thin filament is about 8 nm
diameter, while that of the thick filaments is about 16 nm.
The thick and thin filaments overlap one another to a greater or lesser extent
depending on whether the muscle is contracting or relaxing. The pattern of their
overlap causes the cross-striation seen in single myofibrils and whole-muscle fibers.
The filaments inside a myofibril do not extend the entire length of a muscle fiber.
They are arranged in compartments called sarcomeres.
Narrow plate-shaped regions of dense material called Z-discs (lines) separate
one sarcomere from the next. Within a sarcomere, the darker area, called the A
(anisotropic)-band, extends from one end to the other of the thick filaments where
they overlap the thick filaments. A lighter, less dense area called the I (isotropic)-
band contains the rest of the thin filaments but no thick filaments. The Z-disc passes
through the center of each I-band. The alternating dark A-bands and light I-bands
give the muscle fiber its striated appearance. A narrow H-zone in the center of each
A-band contains thick but not thin filaments. Dividing the H-zone is the M-line
formed by protein molecules that connect adjacent thick filaments.
µ
